(1) Field of the Invention
The present invention relates generally to the inspection of nuclear steam generators and, more particularly, to a retractable probe assembly for placing monitoring devices near areas difficult to access during a chemical cleaning operation, such as a tube support plate of the generator, and removing those devices while the generator is being cleaned under process temperature and pressure.
(2) Description of the Prior Art
A nuclear steam generator typically includes a shell spaced apart from and surrounding an inner shroud. The shroud surrounds the tie rods, tubes, tube support plates and a tube sheet therein. The vessel's tie rods are within and run parallel to the shroud and tubes. Generally, the shell has an opening with a nozzle neck extending therefrom. The nozzle neck is equipped with an annular flange. The opening is usually above a tube support plate or a tube sheet. The shroud includes an opening substantially aligned with the opening of the shell and the nozzle neck.
Because of the build up of corrosion during normal operation, it is necessary to periodically chemically clean and/or monitor corrosion in the generator. The condition of the area around the upper surface of the tube support plate and/or the tube sheet and the inner walls of the shroud near these regions should be monitored during the chemical cleaning process to determine an amount of corrosion to assess the structural integrity, project vessel life and evaluate vessel health.
A major obstacle for such cleaning and monitoring processes is the absence of a probe positioner capable of reaching the tube support plate or tube sheet through the nozzle neck opening without having the monitoring device contact the tie rods or other structures within the shroud. Contacting these structures with a monitoring device may result in damage to the monitoring device and may result in erroneous readings. The difficulty in reaching the tube support plate or tube sheet with the monitoring device is that the monitoring device must be placed through the nozzle neck and across the gap between the shroud and the shell into the inside of the shroud and then drop immediately downward to avoid contact with the tie rods.
The difficulty in placing a monitor near the tube support plate given the above described obstacles was addressed in a patent application for a probe positioner, U.S. Ser. No. 08/679,966, filed Jul. 15, 1996, now U.S. Pat. No. 5,756,908 to Knollmeyer et al. The Knollmeyer et al. patent provides for actively monitoring these hard to reach areas electronically while the generator is off-line.
The Knollmeyer et al. patent does not provide the ability to passively monitor the vessel, with devices such as corrosion coupons, while the generator is on-line. Therefore, there remains a need to place passive monitoring devices in difficult to access areas within the vessel, subject the monitoring devices to the vessel environment at high temperatures and pressure and remove the monitoring devices for inspection while the generator is on-line.
Metal Samples Company of Munford, Ala., has developed a rigid guide structure and an inflexible rod associated with a packing gland for removing coupons while the generator is on-line. However, this system is incapable of placing or removing coupons below the shell nozzle or opening and does not address the problem presented by the barrier placement about the shell. Applicant is unaware of any system capable of maneuvering monitoring devices into difficult to reach areas and providing removal of these devices without breaching the sealed environment of an operating generator or boiler. Thus, there remains a need for a new and improved probe system which is capable of providing for the removal of monitoring devices from hard to access areas within generator tubing while, at the same time, without breaching the pressurized, sealed environment therein.